Fuse structural members are elongated members which must fail at predetermined loads to avoid damage to surrounding mechanical elements, while still maintaining sufficient strength and rigidity to efficiently and safely transmit forces at sub-fuse loads. In compression, the successful design of a fuse load is complicated by the variable degree in bending of the member as the load increases. The failure point and direction of failure is driven by the presence of manufacturing eccentricities and tolerances, material variations and cross sectional shape throughout the length of the member.
Fuse loads are the predetermined loads that cause fuse structural members to fail prior to damaging surrounding elements. Fuse loads in compression are usually accomplished by the use of either a shear pin or spring mechanism. In the case of the shear pin the load that causes the pin to shear in compression will also shear at the same load in tension. In the case of a spring, the assembly does not fail but begins to displace according to Hooks Law once the spring preload is surpassed. Therefore, the spring continues to add force as displacement increases. Both shear pin and spring mechanism fuse loads are expensive to manufacture, add complexity to designs and have limitations to their applications. In weight or fatigue critical applications, such as aerospace, the use of a spring mechanism may be impractical.
Typical rod assembly design for compression relies heavily on Euler and Johnson-Euler equations to determine safe working loads. These equations establish the theoretical lowest compressive loads where columns, as for example structural rod assemblies and struts, can be expected to buckle. A theoretical column with no load eccentricity, subject to an increasing compressive force, would eventually fail due to compressive yielding. This compressive yielding creates a local instability that will eventually lead to bending and subsequent buckling. Initial yielding failure would occur at an unpredictable location and cause instability in an unpredictable direction.
Structural members that experience significant bending during compressive loads may gain structural support if they rest against neighboring elements. According to Euler compressive column theory, a column 10 that gains support 12 (FIG. 1b) at mid span will have a buckling load 8 times higher than column 10 supported by pins 14 (FIG. 1c). A column 10 that gains transverse support at its ends 16 by support members 18 (FIG. 1a) will have a buckling load 4 times higher than a column supported only by pins (FIG. 1c). Even slight support gained from surrounding elements can have large impacts on the magnitude of the expected compressive failure load.
Additionally, failures in pure compression without significant bending rely heavily on compressive yielding. Compressive yielding and the resulting plasticity are extremely hard to accurately predict and can be affected by many factors which are difficult to consistently control in a manufacturing environment. Euler bending failures are described as geometric failures and are not dependent upon the stress capacity of the material.